The structuer and function of the bacterial polypeptide elongation factors Tu and Ts are being studied by the methods of protein chemistry and enzyme kinetics to define the modes of action of the factors in molecular detail. With the aid of the recently determined primary structure of EF-Tu, the binding sites of guanosine nucleotides, EF-Ts, AA-tRNA, kirromycin and ribosomes will be identified and correlated on the three dimensional model of the protein that is now being constructed. To understand how GDP and GTP regulate the function of EF-Tu, differences between the reactivities of various residues in EF-Tu-GDP and EF-Tu-GTP will be explored. A kinetic analysis of the EF-Ts catalyzed reaction will be undertaken to clarify the mechanism of action of this protein.